One of the steps in making glass optical fiber by the vapor axial deposition (VAD) method is the step of sintering or consolidating glass particles or "jacket soot" on a glass core rod. The glass jacket soot is deposited on a glass core rod by flame hydrolysis to form a porous body known as a soot boule. The soot boule is then sintered typically in an atmosphere of helium to form a solid glass clad layer over the glass core rod. After sintering, the clad layer typically contains imperfections due to trapped helium which are removed by annealing the structure in an inert atmosphere of a gas having a greater molecular weight than helium, typically nitrogen. As is pointed out, for example, in the Ito et al., U.S. Pat. No. 4,978,378, granted Dec. 18, 1990, annealing in an inert heavier gas is necessary for driving helium out of the glass clad layer, and nitrogen is the least expensive inert gas available for that purpose. (In this context, nitrogen is customarily treated as an inert gas.)
In our experience, it takes typically about one hundred forty-three minutes to sinter completely the glass soot boule in the helium atmosphere. Complete sintering results in a jacket layer of clear glass that has shrunk in volume about fifteen to twenty-five percent from the original glass boule volume. Annealing is then done typically for one hundred twenty minutes in a nitrogen atmosphere at a temperature that is typically kept below the glass transition temperature, which is about 1400 degrees Centigrade.
Optical fiber is very widely used, and as a consequence significant efforts have been made to reduce production costs. Any measure that would reduce significantly the time taken for sintering and annealing would therefore be regarded as a significant advance in the art.